CD97 was discovered as a result of "reverse genetics" and therefore, relatively little is known about the mechanisms involved in CD97 function. The primary structure suggests a G-protein coupled receptor (GPCR), which utilizes an adhesion class of ligand. We have determined that integrin alpha 5, beta 1 is one ligand for CD97, and conversely CD97 can act as a ligand to stimulate endothelial cell activation via integrin receptor binding. We recently have discovered that ectopic CD97 leads to Rho activation, probably through stochastic activation of the CD97 heptahelical receptor which couples to G?12/13. Rho activation has been shown to have a significant role in cancer cell motility and invasion as a result of regulating amoeboid, bleb-associated motility that is independent of pericellular proteolysis. Therefore, we hypothesize that increased CD97 expression is one mechanism whereby certain cancer cells activate the Rho/ROCK pathway and increase their invasive potential. Because CD97 is a GPCR, it is a good candidate as a target for small molecule inhibitors capable of "locking" CD97 in an inactive state.In order to address the function of CD97 relative to cancer biology, we have used several approaches that assess invasiveness and tumorigenicity. The KAT4 anaplastic thyroid cancer cell line expresses an intermediate level of CD97. We have determined that increasing CD97 expression leads to enhanced motility and invasiveness in vitro. Reducing the level of CD97 greatly diminishes the tumorigenic potential in xenograft assays. Thus, CD97 may be a factor that contributes to the grade of thyroid carcinoma. We are further testing the effect of CD97 expression on the progression of follicular thyroid carcinomas in vivo using transgenic mouse models. Different lines of RET/PTC1 transgenic mice develop follicular thyroid hyperplasia or noninvasive carcinomas. We have developed transgenic strains of mice that overexpress CD97 in thyrocytes. By crossing appropriate transgenic strains, we will evaluate an effect of CD97 overexpression in thyrocytes predisposed to develop thyroid cancer.